[452/906] sin: get rid of hls conversion code

Get rid of buggy and complicated hls conversion code for the sin effect.
The only thing needed was hue anyway and it is easily calculated using
Preucil formula for rgb to polar coordinates conversion.
Now works on i915 (removed all the IF blocks). Still needs some tuning,
I wonder if it will ever work properly.

gst/gl/effects/gstgleffectssources.c

@@ -455,48 +455,27 @@ const gchar *rgb_to_curve_fragment_source =
 const gchar *sin_fragment_source =
   "#extension GL_ARB_texture_rectangle : enable\n"
   "uniform sampler2DRect tex;"
-  "vec3 rgb2hsl (vec3 v)"
-  "{"
-/* TODO: check this algorythm */
-  "  float MIN, MAX;"
-  "  float r, g, b;"
-  "  float h, l, s;"
-  "  float delta;"
-  "  h = 0.0; l = 0.0; s = 0.0;"
-  "  r = v.r; g = v.g; b = v.b;"
-  "  MIN = min (r, min (g, b));"
-  "  MAX = max (r, max (g, b));"
-  "  delta = MAX - MIN;"
-  "  l = (MAX + MIN) / 2.0;"
-  "  if ((MAX - MIN) < 0.0001) { h = 0.0; s = 0.0; }"
-  "  else {"
-  "    if (l <= 0.5) s = (MAX - MIN) / (MAX + MIN);"
-  "    else s = (MAX - MIN) / (2.0 - MAX - MIN);"
-  "    if (r == MAX) h = (g - b) / delta;"
-  "    else if (g == MAX) h = 2.0 + (b - r) / delta;"
-  "    else h = 4.0 + (r - g) / delta;"
-  "    h *= 60.0;"
-  "    if (h < 0.0) h += 360.0;"
-  "  }"
-  "  return vec3 (h, l, s);"
-  "}"
   "void main () {"
-  "  vec3 HSL, RGB;"
   "  vec4 color = texture2DRect (tex, vec2(gl_TexCoord[0].st));"
   "  float luma = dot(color.rgb, vec3(0.2125, 0.7154, 0.0721));"
-  "  HSL = rgb2hsl (color.rgb);"
-/* move hls discontinuity away from the desired red zone so we can use
- * smoothstep.. to try: convert degrees in radiants, divide by 2 and
- * smoothstep cosine */
-  "  HSL.x += 180.0;"
-  "  if ((HSL.x) > 360.0) HSL.x -= 360.0;"
-/* damn, it is extremely hard to get rid of human face reds! */
-/* picked hue is slightly shifted towards violet to prevent this but
- * still fails.. maybe hsl is not well suited for this */
-  "  float a = smoothstep (110.0, 150.0, HSL.x);"
-  "  float b = smoothstep (170.0, 210.0, HSL.x);"
-  "  float alpha = a - b;"
-  "  gl_FragColor = color * alpha + luma * (1.0 - alpha);"
+/* calculate hue with the Preucil formula */
+  "  float cosh = 0.5*(2*color.r - color.g - color.b);"
+/* sqrt(3)/2 = 0.866 */
+  "  float sinh = 0.866*(color.g - color.b);"
+/* hue = atan2 h */
+  "  float tanh = sinh/cosh;"
+/* ok this is a little trick I came up because I didn't find any
+ * detailed proof of the Preucil formula. The issue is that tan(h) is
+ * pi-periodic so the smoothstep thing gives both reds (h = 0) and
+ * cyans (h = 180). I don't want to use atan since it requires
+ * branching and doesn't work on i915. So take only the right half of
+ * the circle where cosine is positive */
+/* take a slightly purple color trying to get rid of human skin reds */
+/* tanh = +-1.0 for h = +-45, where yellow=60, magenta=-60 */
+  "  float a = smoothstep (-1.0, -0.5, tanh);"
+  "  float b = smoothstep (-0.1, 0.4, tanh);"
+  "  float mix = (a - b) * step (0.0, cosh);"
+  "  gl_FragColor = color * mix + luma * (1.0 - mix);"
   "}";
 
 const gchar *interpolate_fragment_source =